1. Field of the Invention
This invention relates to a binder resin usable in toners for developing electrostatic latent images, a toner for developing electrostatic latent images, an image-forming method making use of the toner and an image-forming apparatus making use of the toner. More particularly, it relates to a binder resin, a toner for developing electrostatic latent images, an image-forming method and an image-forming apparatus which are used in electrophotography, electrostatic recording and electrostatic printing performed in copying machines, printers, facsimile machines and so forth, in which a toner image is previously formed on an electrostatic-latent-image-bearing member (hereinafter simply “image-bearing member”) and is thereafter transferred onto a transfer medium to form an image. Still more particularly, it relates to a binder resin which has a biodegradability and at the same time contributes to superior fixing performance (low-temperature fixing performance, fixing temperature characteristics and anti-offset properties) and blocking resistance, and which has hydrolyzability and biodegradability and can readily be deinked, where existing deinking systems can be utilized as they are, also enabling waste disposal with ease; a toner for developing electrostatic latent images which contains such a binder resin; and an image-forming method and an image-forming apparatus which make use of the toner.
2. Related Background Art
A number of methods are conventionally known as methods for electrophotography. In general, copies are obtained by forming an electrostatic latent image on an image-bearing member (photosensitive member) by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a visible image (toner image), transferring the toner image to a transfer medium such as a paper as occasion calls, and then fixing the toner image to the transfer medium by heating and/or pressing. As methods by which the electrostatic latent image is formed into a visible image, cascade development, magnetic brush development, pressure development and so forth are known in the art. Another method is also known in which, using a magnetic toner and a rotary developing sleeve provided with magnetic poles at the core, the magnetic toner is caused to fly from the developing sleeve surface to the photosensitive member surface by the aid of an electric field.
As development methods used when electrostatic latent images are developed, available are a two-component development method making use of a two-component type developer comprised of a toner and a carrier and a one-component development method making use of no carrier and comprised only of a toner.
Now, fine colored particles commonly called a toner are constituted of a binder resin and a colorant as essential components and besides optionally a magnetic material and so forth. Here, the binder resin occupies the greater part of the toner, and hence the physical properties of such a binder resin influence toner's physical properties greatly. For example, the binder resin is required to have delicate hardness and thermal melt properties, and a toner obtained by pulverizing a binder resin having a colorant and so forth dispersed therein followed by classification must show good fluidity without producing any fine powder against a mechanical impact caused by agitation in a developing assembly and also without causing agglomeration of the toner itself. Also, at the time of fixing, the toner must immediately melt at a low temperature and, when melts, the molten toner must show agglomeration properties. Namely, the controlling of binder resin's physical properties enables control of toner's physical properties.
As the binder resin, conventionally used are a styrene-acrylate copolymer, polyester resin, epoxy resin, olefinic resin and so forth. In particular, polyester resin is widely used at present as a resin for toners for heat-roll fixing, because, e.g., it has advantages such that, when melt, it makes toner additives such as carbon black disperse well and is well wettable to transfer paper.
In recent years, from the viewpoint of environmental conservation, it is also of worldwide consciousness how resources be recycled, how waste be curtailed, how the safety of waste be improved, and so forth. Such a subject is not exceptional also in the field of electrophotography. More specifically, with wide spread of copying machines and printers, the disposal of fixed toner on paper, waste toner after use, printed paper, copying paper and so forth is increasing year by year. Here, conventional toners are sparingly degradable because they are constituted of components all of which are stable artificial compounds, and may remain in all environment, e.g., in soil and in water over a long period of time. Also, in order to recycle resources, it is an important subject to regenerate plain paper for its reuse. However, conventional binder resins composed chiefly of styrene resins, it is difficult to remove them from paper surface (deinking) by alkali hydrolysis. This has come to be one of subjects in the recycling of plain paper. The safety of waste is also an important subject from the standpoints of the conservation of global environment and the influence on human bodies.
Under such circumstances, development is being made on resins which are harmless to human bodies and degradable by the action of microorganisms, i.e., biodegradable resins. For example, it has been reported that many microorganisms are capable of producing a biodegradable resin polyhydroxyalkanoate (hereinafter “PHA” when abbreviated) and accumulating it in the cell (“Handbook of Biodegradable Plastics”, Biodegradable-Plastic Institute, K.K. N.T.S., pp.178-197, 1995). It is known that such a PHA can have various composition and structure depending on the type of microorganisms used for its production, the composition of culture medium, the conditions for culturing and so forth. Researches on how to control the composition and structure of the PHA to be produced have hitherto chiefly been made from the viewpoint of the improvement in its physical properties. With regard to the application of such biodegradable resins, too, they have already given reasonable actual results especially in the field of materials for medical use. In the field of agriculture, too, the biodegradable resins have been put into practical use in multifiles, gardening material, sustained-release agricultural chemicals, fertilizers and so forth. In the field of leisure industry, too, the biodegradable resins are used in fishing lines, fishing articles, golf goods and so forth. Besides, as packaging materials for daily necessities, they have been put into practical use in containers or the like of living articles. However, considering their wide application as plastics, under the existing conditions they can not still be said to be satisfactory in respect of physical properties. For example, in order to make the PHA utilizable in much wider ranges, it is important to study the improvement of physical properties more widely. For that end, it is essential to make development and research on PHAs containing monomer units of various structures.
In the field of electrophotography, too, methods in which biodegradable resins are used in binder resins are proposed as methods by which toners which are disposable without causing environmental pollution. For example, Japanese Patent Application Laid-Open No. 6-289644 discloses an electrophotographic toner particularly used for heat-roll fixing, which is characterized in that at least a binder resin contains a vegetable wax and a biodegradable resin (as exemplified by polyesters produced by microorganisms and natural polymeric materials derived from vegetables or animals), and the vegetable wax is added to the binder resin in an amount of from 5 to 50% by weight. Japanese Patent Application Laid-Open No. 8-262796 also discloses an electrophotographic toner containing a binder resin and a colorant, and is characterized in that the binder resin comprises a biodegradable resin (as exemplified by aliphatic polyester resins) and the colorant comprises a water-insoluble coloring matter. Also, U.S. Pat. No. 5,004,664 discloses a toner having as its composition polyhydroxybutyric acid, polyhydroxyvaleric acid, or a copolymer or blend of these. In these techniques, when buried for disposal, the binder resins can certainly be degraded in soil because they are biodegradable. However, there have been problems on fundamental function as binder resins, such that the toner has a low running performance and also is unstably chargeable because of its high moisture absorption. For example, the PHB is a hard and brittle material having properties of a melting point of 180° C., a crystallinity of 50 to 70%, a Young's modulus of 3.5 GPa and a breaking extension of 5%, and is insufficient in practical use for its use as the binder resin of toner.
A toner composed chiefly of a polylactic acid type aliphatic polyester is also proposed as having a biodegradability and also being efficiently degradable in alkali hydrolysis and hence being useful for the recycling of paper. For example, Japanese Patent Application Laid-Open No. 7-120975 also discloses a method of making a lactic-acid homopolymer into a toner, giving as its typical example a polylactic acid obtained by ring-opening polymerization.
In the ring-opening polymerization, a method is employed in which the lactic acid is first made into an oligomer by dehydration reaction, which oligomer is then subjected to depolymerization to lead it to a cyclic dimer lactide and is further subjected to ring-opening polymerization. Since such complicate steps are followed, the resultant polylactic acid comes very highly expensive for its use as a toner resin (a resin for toners).
In addition, since the ring-opening polymerization is cationic ring-opening polymerization, it is necessary, e.g., to make unhydrous the solvent used and to remove any ionic species which may serve as a polymerization terminator, resulting in a poor production efficiency. Moreover, the monomer species that can be used when the polyester is produced is limited to a cyclic ester, and hence it is not easy to control physical properties required as toner resins. It is also difficult to effect copolymerization with various monomers in order to control the balance between degradability and physical properties. In this regard, it is sought to provide a degradable polyester that can control its physical properties inexpensively and with ease. Also, when the polylactic acid is made into a toner as it is, there are problems also on the storage stability and anti-offset properties of the toner. Thus, such a toner has not yet been put into practical use.
Japanese Patent Application Laid-Open No. 9-274335 also discloses a toner for developing electrostatic latent images which is characterized by containing a polyester resin and a colorant; the former being obtained by dehydration polycondensation of a composition containing lactic acid and a tri- or more functional oxycarboxylic acid. However, the polyester resin is formed through dehydration polycondensation reaction of an alcohol group of the lactic acid with a carboxylic group in the oxycarboxylic acid. Hence, it is considered that the resultant resin tends to have a large molecular weight, and therefore has a low biodegradability. Also, like the one disclosed in Japanese Patent Application Laid-Open No. 7-120975, there are problems on the storage stability and anti-offset properties of the toner.
Polycaprolactone, which is a homopolymer of a typical hydroxycarboxylic acid, also has a low melting point and a low glass transition point and has good compatibility with various resins. It, however, has a melting point of as low as 60° C., and is not suitable as a binder resin when used alone. Also, the polylactic acid has a high glass transition point (60° C.), and one having crystallizability is a thermoplastic high polymer having a high melting point (about 180° C.), which, however, has not yet been put into practical use as a binder resin as stated previously. Moreover, toner resins comprised of the conventional degradable polyester resin commonly have so poor a pulverizability that it is difficult for them to be used as binder resins which occupy 90% of toners of about 10 μm in particle diameter. Accordingly, taking account of their practical use as binder resins of toners, it has strongly been desired to improve their physical properties,